1. Field of the Invention
This invention relates to a semiconductor laser pumped solid state laser, and more particularly to a semiconductor laser pumped solid state laser in which generation of noise due to return light to the semiconductor laser is prevented.
2. Description of the Related Art
There has been known a solid state laser in which a solid laser medium is pumped by a semiconductor laser (laser diode) as disclosed, for instance, in Japanese Unexamined Patent Publication No. 6(1994)-69564.
In such a semiconductor laser pumped solid state laser, there has been a problem that return light, which is pumping light reflected at an end face of the laser medium, makes oscillation of the semiconductor laser unstable and fluctuates intensity and/or wavelength of the pumping light, thereby generating noise and/or fluctuation in the output of the solid state laser.
As an arrangement for overcoming the problem, there has been known a structure in which a return light intercepting wavelength plate having a desired phase delay is disposed between the semiconductor laser and the resonator of the solid state laser, as disclosed, for instance, in Japanese Unexamined Patent Publication No. 6(1994)-69564. In this approach, the direction of linear polarization of a pumping light beam returning toward the semiconductor laser is set in perpendicular to the direction of linear polarization of a pumping light beam which is emitted from the semiconductor laser and travels toward the solid laser medium by the return light intercepting wavelength plate, thereby preventing interference of the light beams with each other.
Further there has been proposed an arrangement in which the pumping beam is caused to impinge upon the light inlet end face of the solid laser medium obliquely thereto as disclosed in Japanese Unexamined Patent Publication No. 6(1994)-97545.
However the arrangement disclosed in Japanese Unexamined Patent Publication No. 6(1994)-69564 is disadvantageous in the that a wavelength plate which is generally of rock crystal and expensive is used, which adds to the cost. Further, in this arrangement, when there is some optical element other than the solid laser medium in the resonator of the solid state laser, the phase of the pumping beam reflected at the optical element can differ from that of the pumping laser beam reflected at the end faces of the laser medium. Further when the solid laser medium itself exhibits birefringence like YVO.sub.4, the pumping beam reflected at the light outlet side end face differs from that reflected at the light inlet side end face in phase. The direction of linear polarization of such light beams cannot be perpendicular to the direction of linear polarization of a pumping light beam which is emitted from the semiconductor laser and travels toward the solid laser medium by the return light intercepting wavelength plate. Accordingly it is difficult to completely prevent interference of light beams and generation of noise due to the return light cannot be sufficiently suppressed.
Further, in the arrangement disclosed in Japanese Unexamined Patent Publication No. 6(1994)-97545, since the light emitting surface of the semiconductor laser and the reflecting surface of the solid laser medium form a confocal optical system, return light entering a lens of a light entrance optical system is all converged on the light emitting surface of the semiconductor laser. Accordingly in order to suppress generation of noise due to the return light, it is necessary for the light entrance optical system to be inclined at such a large angle that return light cannot enter the lens.
However when the light entrance optical system is inclined at such a large angle, the effective beam diameter of the pumping beam in the solid laser medium becomes large as can be understood from FIGS. 3A and 3B where reference numerals 1 and 2 respectively denote the solid laser medium and the pumping beam, and matching of the pumping laser beam with the solid laser beam. As a result, there arise problems of deterioration in efficiency and fluctuation in output power and generation of noise due to deterioration of transverse mode. Further when the light entrance optical system is inclined at such a large angle, the overall size of the solid state laser is increased.